Twig Query Processing Under Concurrent Updates

An appropriate database language characteristics leading to the success of declarative query processing - and, in turn, to the rise of relational DBMSs in general - always provides more than one way of evaluating a query. This counts for structurally different but logically equivalent query evaluation plans (QEPs) as well as for different implementations of the same logical operator. This principle surely holds for the novel XML database management systems (XDBMSs): Recently proposed operators for XML query processing can be grouped into the logical operators Structural Join [1, 22] and Holistic Twig Join [3, 6, 16]. Depending on available internal system mechanisms, a lot of opportunities exist how to implement these operators (two of which are presented in this paper.